Fast fading channels can present dynamic range challenges for symbol based wireless systems, such as orthogonal frequency division multiplexing (OFDM), orthogonal frequency division multiple access (OFDMA), and time division duplex (TDD) systems with frame-based AGC. Problems exist because signal strength may fluctuate significantly during a frame which contains multiple symbols, resulting in differing signal to noise ratios (SNRs) among symbols. Further compounding the challenges are differing SNR requirements for various common modulation schemes.
For example, 16 quadrature amplitude modulation (16 QAM) will have poorer bit error rate (BER) performance than quadrature phase shift keying (QPSK) for a given SNR. To compensate for this difference, the SNR requirement for 16 QAM is higher than it would be for QPSK. For a system that uses multiple modulation schemes, meeting a minimum performance level may require significant dynamic range in order to accommodate the differing SNR requirements over fading channels.
In order to maintain an acceptable SNR within a system's dynamic range, receivers often employ AGC. Traditionally, AGC circuits control the gain of a variable gain amplifier to maintain an amplified signal within a target power range. By comparing the amplified signal power to a reference, a determination can be made as to whether the amplifier should be adjusted to provide more gain or less gain. A control signal then causes the gain adjustment. Since the feedback loop has a time lag, if power adjustments are made too quickly, the feedback loop may become an oscillator which could introduce a ping-pong effect in the power level. To combat this, TDD-OFDM systems often adjust gain on a relatively slow, frame-by-frame basis, presenting the above-mentioned problem of differing symbol SNRs.